Engineering Nanoscale Iron Oxides for Uranyl Sorption and

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Engineering Nanoscale Iron Oxides for Uranyl Sorption and Separation: Optimization of Particle Core Size and Bilayer Surface Coatings Wenlu Li, Lyndsay D. Troyer, Seung Soo Lee, Jiewei Wu, Changwoo Kim, Brandon J Lafferty, Jeffrey G Catalano, and John D. Fortner ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01042 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on April 1, 2017

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ACS Applied Materials & Interfaces

Engineering Nanoscale Iron Oxides for Uranyl Sorption and Separation: Optimization of Particle Core Size and Bilayer Surface Coatings Wenlu Li,＃ Lyndsay D. Troyer,§ Seung Soo Lee,＃ Jiewei Wu, ＃ Changwoo Kim, ＃ Brandon J. Lafferty,‖ Jeffrey G. Catalano, § and John D. Fortner*, ＃＃

Department of Energy, Environmental, and Chemical Engineering,

Washington University in St. Louis, St. Louis, MO, 63130, United States §

Department of Earth and Planetary Sciences,

Washington University in St. Louis, St. Louis, MO, 63130, United States ‖

U.S. Army Corps of Engineers, Engineer Research and Development Center, Vicksburg, MS, 39180, United States

*To whom correspondence should be addressed: John D. Fortner: Tel: +1-314-935-9293; Fax: +1-314-935-5464; Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT

Herein, we describe engineered superparamagnetic iron oxide nanoparticles (IONPs) as platform materials for enhanced uranyl (UO22+) sorption and separation processes under environmentally relevant conditions. Specifically, monodispersed 8-25 nm iron oxide (magnetite, Fe3O4) nanoparticles with tailored organic acid bilayered coatings have been systematically evaluated and optimized to bind, and thus remove, uranium, from water. The combined non-hydrolytic synthesis and bilayer phase transfer material preparation methods yield highly uniform and surface tailorable IONPs, which allow us to directly evaluate the size-dependent and coatingdependent sorption capacities of IONPs. Optimized materials demonstrate ultra-high sorption capacities (>50% by wt/wt) at pH 5.6 for 8 nm oleic acid (OA) bilayer and sodium monododecyl phosphate (SDP) surface stabilized IONPs. Synchrotron X-ray absorption spectroscopy shows that iron oxide core particle size and stabilizing surface functional group(s) substantially affect U(VI)-removal mechanisms, specifically the ratio of uptake via adsorption versus reduction to U(IV). Taken together, tunable size and surface functionality, high colloidal stability, and favorable affinity towards uranium provide distinct synergistic advantage(s) for the application of bilayered IONPs as part of next generation material-based uranium recovery, remediation, and sensing technologies.

KEYWORDS

Iron oxide nanoparticles (IONPs), nanoparticle stability, bilayer surface coating, critical coagulation concentration, uranium sorption, uranium reduction, environmental remediation, XAFS


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INTRODUCTION Uranium remains a relevant environmental contaminant by way of mining and refining of uranium ores, nuclear testing, waste disposal, and accidental spills, among other exposure pathways.1-4 For many of these systems, hexavalent uranyl (UO22+) is the most stable and predominant uranium complex found in aerobic, oxic environments.5-6 Because of its significant solubility, U(VI) can easily enter the food/water chain and thus pose high risk to human health,3, 7-9

and the United States Environmental Protection Agency (U.S. EPA) has established the

maximum contaminant level (MCL) for uranium to be at 30 ppb (µg/L) in groundwater (2003).1012

In order to meet this regulation and others, a number of treatment technologies have been

explored for the remediation / removal of uranium species in water(s), including ion-exchange, solvent extraction, membrane filtration, adsorption, and reductive precipitation processes.13-16 Among those, sorption has been found to be an effective and economic method with high potential for sensing, removal, recovery, and potential recycling of uranium from wastewaters / seawaters. In recent years, numerous organic and inorganic adsorbents (e.g. activated carbon, mesoporous silica, polymers, zeolites, and metal/metal oxides) have been evaluated for their affinity towards uranium species.1, 5, 13-15, 17-21 In particular, iron oxide core nanoparticles as a technology platform for uranium separation and low-level detection in (lightly) contaminated environmental media,2,

10-11, 22

offer a number of advantages including low-field (< 1T)

(superpara)magnetic control (e.g. Fe3O4, magnetite), large surface area to volume ratios, and favorable surface enthalpies.18, 23-26 While considerable efforts have been made to develop novel iron oxide materials for the remediation of uranium, further optimization is still necessary to achieve optimum dispersivity and sorption capacities.


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To date, a number of synthetic procedures for nanoscale iron oxide particles have been well established, including co-precipitation, microemulsion, thermal-decomposition, hydrothermal, sonochemical, and gas phase synthesis (e.g. chemical vapor deposition, CVD) methods.27-28 Among these processes, thermal-decomposition based methods provide highly uniform nanoparticles

with

superior

control

over

particle

sizes,

shapes,

and

surface

chemistries/coatings.28 As such materials are usually prepared and stabilized in high temperature organic solvents (e.g. 1-octadecene) and thus surface modification to change the particle surface property from hydrophobic to hydrophilic is required for aqueous based environmental applications.29 In terms of hydrophilic surface coatings, both inorganic shell and organic layer have been widely reported.30-31 For heavy metal ion removal, the stabilization shell with specific functionalities which provide extra sorption sites is highly preferred.25, 32 The overall goal of this study is to develop and optimize nanoscale, superparamagnetic IONPs with soft coatings to stabilize core materials in water - while optimizing surface to volume ratios and enthalpies of analyte interaction(s), as versatile platform materials to sorb uranium for advanced sensing and separation application(s). Here, uranium sorption capacity is evaluated as a function of particle core size, bilayered surface coatings, and pH, under environmentally relevant water chemistries. IONPs were developed, synthesized, and characterized, with sizes ranging from 8 to 25 nm as stable, monodispersed aqueous suspensions through a number of organic acid surface bilayers (systematically varying effective surface chemistries). Particle stabilities were characterized and described by ionic strength and cation type using time resolved dynamic light scattering (TR-DLS). Uranyl sorption capacities, via isotherm studies, were quantitatively defined as a function of particle core sizes, surface coating, and aqueous chemistries. Sorption mechanism(s) and resulting sorbed species were explored and described using X-ray absorption


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spectroscopy. Among the expanded series of particle core compositions and surface chemistries (and bilayer structures) evaluated, stable IONPs with terminal carboxyl and phosphate functionalities demonstrate uranium sorption capacity of >0.5 g of U per g of Fe, which is the highest reported uranium sorption capacity for any iron oxide (core) particle to date.2,

10, 33

Results presented provide fundamental insight into the mechanisms of uranium association with these novel materials and the molecular-scale coordination of the solid-associated uranium species, thus allowing for continued refinement of these materials for sensing and even remediation applications. MATERIALS AND METHODS Materials: Magnetite (catalog # 637106, iron (II, III) oxide, < 50 nm, 98+%), iron(III) oxide (hydrated, catalyst grade), 1-octadecene (technical grade, 90%), oleic acid (technical grade, 90%), sodium chloride (ACS reagent, ≥99.0%), calcium chloride dihydrate (ACS reagent, ≥99%), sodium hydroxide (ACS reagent, ≥97.0%) , oleic acid (OA, 99%), sodium stearate (SA, 99.0%), sodium laurate (LA, 99%), sodium monododecyl phosphate (SDP), sodium dodecyl sulfate (SDS, 99.0%), N-Lauroylsarcosine sodium salt (NLS, 97%), nitric acid (trace metal grade), and reagent grade of hexane, acetone, and ethanol were all obtained from Sigma-Aldrich (Saint Louis, MO, USA). Uranyl nitrate hexahydrate (UO2(NO3)2•6H2O) was purchased from Antec, Inc. (Louisville, KY, USA). All chemicals were used as received without any further purification. Synthesis and Characterization of IONPs: Detailed synthesis and phase transfer of IONPs can be found elsewhere.28-29, 34 Briefly, highly uniform IONPs were first synthesized via high temperature (ca. 320 °C) decomposition of iron carboxylate salts in the mixture of oleic acid and 1-octadecene. Depending on the target material size, various amounts of precursors were mixed


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in the reaction system. The product was purified with a standard hexane/acetone procedure and the purified IONPs were then collected in hexane. The concentration of the nanoparticle core material (Fe) was measured by inductively coupled plasma mass spectrometry (ICP-MS, Agilent 7500ce) after acid digestion (10% nitric acid). Transmission electron microscopy (TEM, FEI Tecnai G2 Spirit) was used to characterize the IONPs size. Mean particle size and size distribution were then analyzed by ImageJ software package (National Institutes of Health). The obtained IONPs were then phase transferred from the hexane to water following the procedure previously reported.34 In a 20 mL glass vial, the mixtures of surfactants, IONPs in hexane, and water were intensively mixed by probe sonication (UP 50H, Dr. Hielscher, GMHB). After settling and the removal of trace amounts of hexane, the colored aqueous phase was collected and purified for further use. To determine the aqueous Fe mass concentration, water stable IONPs were digested by strong nitric acid on a hot plate and analyzed for their Fe concentration with ICP-MS. The hydrodynamic diameter and zeta (ζ) potentials of transferred IONPs were determined by dynamic light scattering (Malvern Nano ZS, UK). Mean value and standard deviation of diameter and zeta potential were calculated based on 10 measurements using at least three separate samples. The aggregation kinetics of bilayered IONPs were conducted in the presence of various concentrations of NaCl and CaCl2 using time-resolved dynamic light scattering, following previously reported procedures.29, 34-35 Uranium Sorption Isotherm Measurements: In methods similar to these prior published studies, batch experiments for uranium sorption were conducted using bilayered IONPs with four different sizes and six types of surface coating.12, 36 A stock solution of U(VI) was prepared by dissolving UO2(NO3)2•6H2O in acidified ultrapure water (pH 3). Aqueous solutions at various uranium concentrations over a pH range from 5-9 were mixed with IONPs suspensions (total 10


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mL, 0.2 mg of iron) in test tubes (polypropylene, 15 mL, VWR). Solution pH was maintained by the addition of negligible volumes of NaOH or HNO3 (0.1 M). The test tubes were put on an end-over-end shaker for 24 hours, which is approximately twice the observed equilibrium time.12 All experiments were performed under room temperature (21.0 ± 1.0 oC) and atmospheric conditions. In control experiments, uranium sorption onto polypropylene/polycarbonate testtube/centrifuge tube walls was carried out under the same experimental procedures without the addition of IONPs. All experiments were conducted in triplicate. Upon equilibrium, the suspensions were separated and analyzed by ICP-MS to determine the amount of uranium remaining in solution. The sorption capacity, q, will be determined by dividing the amount of uranium sorbed onto the IONPs by the mass of the IONPs. The measured uranium sorption capacity (mg/g) of IONPs as a function of equilibrium uranium concentration (mg/L) was fitted using the Langmuir isotherm (equation 1) and the Freundlich isotherm (equation 2): (1) (2) (3) where

is the amount of adsorbed uranium at equilibrium concentration (mg/g),

sorption constant (L/mg),

is the

is the maximum sorption density of the solid (mg/g; mass of the

sorbed uranium per mass of Fe), K is the Freundlich constant indicative of adsorption capacity ((mg/g)(L/mg)1/n), n is the Freundlich constant representing adsorption intensity, and corresponds to the equilibrium concentration of uranium (mg/L) as determined by ICP-MS. Sorption density

can be calculated from equation (3), where

is the initial concentration of

uranium (mg/L), V is the volume of the solution (L), and m is the weight (mg) of Fe from IONPs.


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X-ray Absorption Fine Structure Spectroscopy (XAFS): We conducted a straightforward matrix of experiments to spectroscopically explore, via XAFS, the U(VI) coordination and speciation at the interfaces of these bilayered IONPs. Information of the specific coordination environment of adsorbed uranium to either the iron oxide surface or the organic molecules (at the surface) is technically important to understand when trying to maximize/stabilize binding capacities. To prepare the samples for XAFS analysis, 5 mg (as Fe) of commercial available magnetite (Sigma-Aldrich) along with engineered IONPs of various sizes and coatings were mixed with 100 mL dissolved U(VI) (100 mg/L, as depleted uranyl nitrate) and equilibrated for 24 h. Upon equilibrium, the IONPs were filtered and deposited onto an alumina based filter (Anopore, 0.02 um pore size). The filters were then mounted in recesses in custom-made polycarbonate sample holders, followed by wrapping with Kapton tape and heat-sealed under vacuum in polypropylene bags. XAFS samples were then shipped to the Advanced Photon Source (APS) at Argonne National Laboratory (Chicago, USA) and U LIII-edge XAFS spectra were collected on both beamlines 10-BM-B and 12-BM-B. XAFS analyses were performed in fluorescence mode using an Ar-filled ion chamber. Multiple sample scans were collected and averaged for the data analysis. Data were processed in the Athena37 interface to IFEFFIT;38 linear combination fitting of X-ray absorption near-edge structure (XANES) spectra was also conducted in Athena. Structural models were refined versus extended X-ray absorption fine structure (EXAFS) spectra in SixPACK39 using backscattering phase and amplitude functions generated from FEFF 7.0217 as described previously.46 Reference spectra of the U(VI) adsorbed to hematite and the synthetic UO2 were taken from prior studies.40-41 RESULTS AND DISCUSSION Synthesis and Characterization of IONPs


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Highly uniform, spherical IONPs of 8 nm (8.1 ± 0.7 nm), 15 nm (15.1 ± 1.3 nm), 20 nm (19.3 ± 1.4 nm), and 25 nm (23.9 ± 2.3) were serially synthesized and evaluated for this study, as shown in the TEM micrographs (Figure 1). Similar to results previously reported,28 synthesized nanoparticles exhibit relatively narrow size distribution as illustrated in Figure S1. The size of IONPs can be precisely controlled by changing the concentration of initial iron precursor and oleic acid while maintaining solvent volumes.36 When FeO(OH) is increased from 2 mmol to 10 mmol, the diameter of the synthesized IONPs can be increased from 8 nm to 25 nm, which has been observed and reported previously for other metal oxide NPs prepared similarly.27-28,

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IONPs prepared from this non-hydrolytic method are initially capped by an oleic acid monolayer and thus stabilized in non-polar solvents (in this case, hexane). For as-synthesized particles, a water stabilizing, surface bilayer is achieved by mixing a small amount of additional surfactant (oleic acid, stearic acid, lauric acid, N-Lauroylsarcosine, sodium dodecyl sulfate and sodium monododecyl phosphate for this study, as depicted in Table S1) with ultrapure water and the IONPs in hexane (creating a liquid-liquid two-phase system), and then using a sonication probe to facilitate the phase transfer process.29 A stable, homogenous bilayer is formed when the hydrophobic tail of the outer layer aligns with the original hydrophobic tail of the first oleic acid layer (via hydrophobic and dispersive Van der Waals forces), and the hydrophilic head group (e.g. carboxylic acid, phosphate, sulfate, etc.) of the second layer aligns towards/at the aqueous interface, rendering the IONPs water stable. Corresponding TEM micrographs (Figure S2) along with DLS measurements (Figure 2), which are discussed below, confirm aqueous transferred IONPs remain monodisperse without core size change. The numberweighted hydrodynamic diameter of the oleic acid bilayer coated IONPs were measured to be 15.6 ± 1.3 nm, 23.2 ± 2.7 nm, 28.0 ± 1.7 nm, and 33.5 ± 1.7 nm for 8 nm, 15 nm, 20 nm, and 25


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nm IONPs originally dispersed in hexane, respectively (Figure S3). The hydrodynamic diameters of stearic acid (SA), lauric acid (LA), N-Lauroylsarcosine (NLS), sodium dodecyl sulfate (SDS) and sodium monododecyl phosphate (SDP) coated IONPs with 8 nm core size were 18.8 ± 2.1 nm, 19.1 ± 2.4 nm, 21.0 ± 1.0 nm, 19.5 ± 1.9 nm, and 20.1 ± 1.4 nm, respectively (Figure 2 and Figure S4). The DLS value for 8 nm bilayered IONPs is consistent with the value reported before, which takes into account the thickness of the bilayer and sorbed water.29,

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All DLS size

measurements reveal a thin and compact bilayered structure of (un)saturated organic acids over the IONPs regardless of particle core size and coating type. Zeta potential analyses show the bilayered IONPs exhibit negative charge from pH 5.6 to pH 8.5 (Table S2). For additional materials characterization and sorption experiments, aqueous bilayered IONPs were freshly prepared and used within three months; no significant change in hydrodynamic diameter and no precipitation of NPs were observed for up to one year. Uranium Sorption Water-stable, bilayer coated IONPs were evaluated for uranium sorption capacity as a function of uranium concentrations over a range of pH values (pH 5.6 - 8.5). For comparison, both Langmuir and Freundlich models were used to fit experimental sorption data (Tables 1 and 2).12, 25

Correlation coefficients (R2) indicate that a Langmuir model better describes sorption

isotherms, rather than a Freundlich model approach, for all systems.2 Calculated uranium adsorption capacity of 8nm OA-IONPs, as a function of pH, is presented in Figure 3. The maximum uranium adsorption capacity (qmax), as calculated using the Langmuir isotherm model, was 635 mg U/g Fe for 8 nm OA-IONPs at pH 5.6. When the pH was adjusted to neutral (pH 7.0) or slightly basic conditions (pH 8.5), the qmax decreased to 454 and 389 mg U/g Fe, respectively. Similar pH based behaviors were observed for all bilayered IONPs tested, regardless of different


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sizes and coatings (Tables 1 and 2). The higher sorption capacity of bilayered IONPs towards uranium at lower pH is attributed to the favorable electrostatic attractions between positively charged uranium species (such as UO22+, UO2(OH)+, and (UO2)3(OH)5+, Figure S5) and the relatively negatively charged IONPs (Table S2).29, 44-46 At neutral or basic conditions (pH > 7), negatively charged U species, such as UO2(OH)3- and/or UO2(CO3)34-, become dominant species (Figure S5).47 Thus, decreased uranium sorption capacities are expected due to electrostatic repulsive interactions between negatively charged IONPs and uranium species.45-46 Additionally, complexation between U(VI) and carbonate ions at higher pH likely limit U(VI) adsorption onto as-evaluated IONPs.47 The effect of IONPs core size and surface coating on uranium sorption capacity is illustrated in Figure 4. When normalized for bilayer coating structure (OA bilayer in this case), 8 nm OAIONPs exhibited the highest qmax of 635 mg U/g Fe at pH 5.6 compared to OA-IONPs with diameters of 15, 20 and 25 nm. The maximum adsorption capacities for 15 nm, 20 nm and 25 nm OA-IONPs for these conditions were 412, 325, and 260 mg U/g Fe, respectively. The lowered sorption capacity of larger IONPs is the consequence of decreased specific surface area. 36 The coating structures (double bond, chain length, and functional group) of the second layer also play an important role in determining the uranium sorption capacity. Oleic acid is an unsaturated (a C8-C9 cis double bond) fatty acid with an 18-carbon chain terminated with a carboxyl head group, while stearic acid is a saturated fatty acid with the same carbon number and a carboxyl head group. Batch sorption experiments show unsaturated (OA)-unsaturated (OA) bilayered IONPs have a maximum sorption capacity of 635 mg U/g Fe, while the uranium sorption capacity of unsaturated (OA)-saturated (SA) bilayer IONPs is only 419 mg U/g Fe. Higher uranium sorption capacity of OA-IONPs is due, in part, to the enhanced particle colloidal


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stability, which will be discussed in detail below. When lauric acid (LA), a saturated C-12 carbon chain carboxylic acid, was used as the second layer, the uranium sorption capacity of LAIONPs is further decreased to 274 mg U/g Fe. The even lower sorption capacity of uranium onto LA-IONPs compared to that of SA-IONPs (419 mg U/g Fe) indicates the importance of second layer chain length (thus stabilizing van der Waals forces) to the net sorption process. Further, a series of saturated fatty acids (LA, NLS, SDS, and SDP) differing in functional (head) group, but with the same chain length, were applied and evaluated for their sorption affinity towards uranium (Table 2 and Figure 4). For these, phosphonate head groups (SDP-IONPs) have the highest maximum uranium sorption capacity, 657 mg U/g Fe, compared to IONPs coated with other functional groups, which was expected based on well documented U-phosphate interactions.11,

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In comparison, IONPs with sulfate head groups (SDS-IONPs) exhibits the

lowest uranium sorption capacity (218 mg U/g Fe) amongst all bilayered IONPs investigated. The bilayered IONPs demonstrated here show much improved uranium sorption capacities compared to other reported iron oxide based (nano)materials under similar experimental conditions (see Table 3). We hypothesize that there is an enhancement (or synergy) when the stabilizing bilayer is present, which is due in part to favorable functionalities at the hydrophilic “tails” of bilayer constituents (e.g. carboxylic acid group for oleic acid, and phosphate acid group for sodium monododecyl phosphate). Colloidal Stability of Bilayered IONPs Equilibrated hydrodynamic diameters of bilayered IONPs, in the presence of varied uranyl concentrations (from 0.1 to 60 mg/L), are shown in Figure 5, Figures S6 and S7. For all pH conditions evaluated, OA-, SA-, and SDP- coated IONPs maintain high stabilities even in the presence of high uranyl concentration (e.g. 60 mg/L, 0.25 mmol). However, LA-, NLS-, and


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SDS- coated IONPs showed significant destabilization as uranyl concentration increased. In the case of SDS-coated IONPs, effective particle size increases from 20 nm to ca. 150 nm under the highest uranium concentration at pH 5.6. To further explore stability behavior, aggregation kinetics for bilayered IONPs were conducted in the presence of salts (Na+ and Ca2+) using timeresolved DLS. Figure S8 shows the aggregation profile of NLS-IONPs in the presence of NaCl and CaCl2. The aggregation rate of NLS-IONPS is observed to increase with increasing NaCl concentrations below 50 mM. However, at higher NaCl concentrations (80 and 200 mM), an increase in electrolyte concentration has no effect on the aggregation rate. Also, we observe divalent Ca2+ has a greater effect on aggregation mechanisms than monovalent Na+, even at low concentrations.34 For comparison purposes, we established and compared the critical coagulation concentrations (CCC) for these particle systems. CCC is defined at the extrapolated point whereby the aggregation of NPs induced by the addition of salt just reaches the maximum aggregation rate. The CCC values are in the order of OA-IONPs (710 mM of NaCl and 10.6 mM of CaCl2), SA-IONPs (452 mM of NaCl and 9.3 mM of CaCl2), SDP-IONPs (250 mM of NaCl and 3.6 mM of CaCl2), NLS-IONPs (51 mM of NaCl and 1.2 mM of CaCl2), SDS-IONPs (45 mM of NaCl and 1.4 mM of CaCl2), and LA-IONPs (16 mM of NaCl and 0.53 mM of CaCl2) based on the calculations.29 Unsaturated oleic acid (C8-C9 double bond) bilayer surface coated IONPs have the highest CCC values of all 6 types of bilayered IONPs studied. Further, these CCC values of bilayered IONPs are in line with the colloidal stability of these NPs in the presence of uranium. As a corollary, the DLS measurements demonstrate uranyl sorption will affect colloidal stability of the bilayered IONPs system, which must be understood for slow (e.g. large volume, long distance) sensing/remediation applications. Such insight may allow for


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additional engineering control and material expansion – thus further optimizing these and other similar materials for such applications. XAFS Analyses of U(VI) Removal Mechanisms To elucidate the effect of particle size and bilayer coating on uranium sorption, XAFS analyses were employed to better understand the specific chemical mechanisms responsible for uranium uptake onto the IONPs. U(VI) reduction and U(VI)-ligand interactions were specifically investigated. XANES spectra were modeled to determine the extent of U(VI) reduction based on nanoparticle core size and bilayer type. Information regarding the structural environment of the bilayer-associated/coordinated uranium (such as near-neighbor atoms, interatomic distances, and coordination numbers) is provided via EXAFS spectra fitting.24, 48 The edge positions and shapes of the X-ray absorption near-edge structure (XANES) spectra for the 15, 20, and 25 nm OA-IONPs are similar (Figure 6). Linear combination fitting of the XANES spectra reveals that U(VI) adsorbs without undergoing measurable reduction on these samples. EXAFS spectra (Figure 7) show that U(VI) is bound to the uncoated particles (commercial magnetite) as an inner-sphere surface complex and that the addition of the oleic acid coating alters the first coordination shell of U(VI). The structural change of the equatorial oxygen shell, the addition of a carbon neighbor, and the substantial decrease in neighboring Fe atoms (Table 4) indicate substantial complexation of uranium by the coating (oleic acid bilayer structure). In contrast, the 8 nm OA-IONPs have different spectral characteristics, with the edge position of the XANES spectrum shifted to lower energy (Figure 6), indicating substantial uranium reduction. The reduction of U(VI) is ascribed to the coupled oxidation of structural Fe(II) from the IONPs, as described in prior studies.2, 23, 49 Linear combination fitting of these spectra with the spectra of UO2 and U(VI) sorbed to hematite indicates that 60 ± 2% is U(IV) for


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the 8 nm OA-IONPs. Fine structure features in the spectra of the 8 nm IONPs are not well reproduced, suggesting that U(IV) does not occur as a UO2-like solid. The weak second-shell features in the Fourier transform EXAFS spectrum of the 8 nm sample (Figure 7) further shows that U(IV) has not precipitated as a crystalline solid and instead likely exists as monomeric U(IV). This form of a uranium reduction product has recently been observed in other systems with complexing ligands or microbial extracellular polymeric substances.50-58 A structural model was not fit to the 8nm OA-IONPs spectra because the samples contain a mixture of U(IV) and U(VI) species. XANES fitting of 8 nm IONPs with various surface coatings, including OA, SA, LA, NLS, SDS, and SDP, indicates that the extent of U(VI) reduction is strongly dependent on the coating type (Figure 8). Partial reduction to U(IV) is observed with all the coatings, ranging from 15% U(IV) with SDS-IONPs to 60% U(IV) with OA-IONPs (Table 5). As discussed above, the different surface coating structure can directly affect the colloidal stability of these bilayered IONPs, thus changing the possible diffusion and contact pathway of U(VI) to the surface of nanoparticles. We observe, in general, higher reduction ratio of the U(VI) in the more stable bilayered IONPs system, such as OA-IONPs. The observations demonstrate the importance of organic stabilizing agents in the contribution to uranium sorption, which is useful in improving the removal efficiency of uranium using surfactant-coated nanoparticles in future remediation strategies. Conclusions In summary, we have successfully demonstrated surface stabilized, size controlled monodisperse IONPs (8-25 nm) for enhanced uranium sorption, separation, and thus concentration in water. Both particle core size and surface coating structure play critical roles in sorption processes as uranyl sorption capacities are dependent on particle size, pH, and bilayer


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types. OA-IONPs and SDP-IONPs demonstrate the highest uranium sorption capacities, 635 and 657 mg U/g Fe, respectively. XANES analyses indicate partial uranium (VI) reduction to uranium (IV) as a function of particle core size and surface coating structure/functionality. The relatively highest uranium reduction was observed on 8 nm OA-IONPs while the lowest reduction percent of uranium(VI) was found on IONPs coated with SDS. EXAFS spectra reveal substantial complexation of uranium with the coatings, indicating the importance of the organic layer(s) for enhanced sorption capacity. In our prior study, we have demonstrated relative high mobility of 8 nm OA-IONPs under environmentally relevant conditions (even at high ionic strength) and their potential for in-situ transport and sensing in the subsurface.34 In combination with high sorption affinity, delivery of such materials to uranium-contaminated soil and groundwater could immobilize U(IV) through adsorption and reduction to less mobile U(VI).49 For ex-situ remediation applications, optimized nanoparticle platform sorbent technologies may be feasible in suspension-based reactor configurations for advanced (magnetic) separation processes. In this context, IONPs with sizes above 12 nm are desired for their effective removal and recovery for low(er) magnetic fields.26 An optimized nanoparticle-based reactor system for low energy, high efficiency separation and recovery of dissolved metals/metalloids is currently under development in our lab. Presented work highlights the potential application of engineered bilayer coated IONPs such as these, as platform, core materials for the sorption, separation, and sensing of uranium, and potentially other elements in the actinide series, within aqueous matrixes.


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Figure 1. TEM micrographs of the as-prepared (a) 8.1 ±0.7 nm, (b) 15.1 ±1.3 nm, (c) 19.3 ±1.4 nm, and (d) 23.9 ±2.3 nm IONPs in hexane. All scale bars are 50 nm.


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Figure 2. Hydrodynamic diameters of OA bilayer coated 8 nm, 15 nm, 20 nm, and 25 nm IONPs as well as OA, SA, LA, NLS, SDS, and SDP coated IONPs with 8 nm core size. Each data point represents the average of 10 measurements from triplicate samples, where error bar indicates the standard deviation.


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Figure 3. pH dependent uranium sorption isotherms of 8 nm OA-IONPs at pH 5.6, pH 7.0 and pH 8.5. Scattered points represent experiment data, the solid line represents the Langmuir model and the dashed line represents the Freundlich model. Each data point represents the average value of triplicate measurements, where error bar indicates the standard deviation.


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Figure 4. Adsorption isotherm of uranium as a function of (a) size: 8 nm, 15 nm, 20 nm and 25 nm OA bilayer coated IONPs and (b) bilayer coatings: OA, SA, LA, NLS, SDS, and SDP coated IONPs. Scattered points represent experiment data, the solid line represents the Langmuir model and the dashed line represents the Freundlich model. Each data point represents the average value of triplicate measurements, where error bar indicates the standard deviation.


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Figure 5. Hydrodynamic diameter change of OA, SA, LA, NLS, SDS, and SDP coated 8 nm IONPs as a function of initial uranium concentration (0.1-60 mg/L) at pH 5.6 after 24 h. Each data point represents the average of triplicate samples, where error bar indicates the standard deviation.


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Figure 6. (A) XANES spectra of (a) U(VI) adsorbed to hematite, (b) U(VI) reacted with 25 nm OA-IONPs, (c) U(VI) reacted with 20 nm OA-IONPs (d) U(VI) reacted with 15 nm OA-IONPs (e) U(VI) reacted with 8 nm OA-IONPs (f) synthetic UO2. (B) 1st derivative XANES spectra with respect to the maxima positions for U(IV) and U(VI). (C) Linear combination fit of the XANES spectrum of the (e) 8 nm OA-IONPs to a mixture of adsorbed U(VI) and UO2.


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Figure 7. Data (dotted) and structural model fits (solid) (with the exception of 8 nm IONPs) to the EXAFS spectra (A) and corresponding Fourier transform magnitudes (B) of U(VI) reacted with (a) 25 nm OA-IONPs, (b) 20 nm OA-IONPs, (c) 15 nm OA-IONPs, (d) 8 nm OA-IONPs, and (e) uncoated commercial IONPs (Sigma-Aldrich).


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Figure 8. (A) XANES spectra of (a) U(VI) adsorbed to hematite, (b) U(VI) reacted with 8 nm OA-IONPs, (c) U(VI) reacted with 8 nm SA-IONPs, (d) U(VI) reacted with 8 nm LA-IONPs, (e) U(VI) reacted with 8 nm NLS-IONPs, (f) U(VI) reacted with 8 nm SDS-IONPs, (g) U(VI) reacted with 8 nm SDP-IONPs, (h) synthetic UO2. (B) 1st derivative XANES spectra with respect to the maxima positions for U(IV) and U(VI). (C) Linear combination fit of the XANES spectrum of (b) 8 nm OA-IONPs, (c) 8 nm SA-IONPs, (d) 8 nm LA-IONPs, (e) 8 nm NLSIONPs, (f) 8 nm SDS-IONPs, and (g) 8 nm SDP-IONPs to a mixture of adsorbed U(VI) and UO2.


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Table 1. Parameters and correlation coefficients for OA bilayer coated IONPs with different sizes derived by Langmuir and Freundlich isotherm models. Langmuir Samples

8 nm

15 nm

20 nm

25 nm

pH

qmax

Freundlich

k R2

K ((mg/g)(L/mg)1/n)

n

R2

(mg/g)

(L/mg)

5.6

635.2

0.2733

0.9812

180.2

2.987

0.8584

7.0

454.3

0.8691

0.9364

185.8

3.830

0.8526

8.5

389.1

1.333

0.9619

187.5

4.649

0.8442

5.6

411.6

0.1663

0.9691

96.70

2.771

0.8433

7.0

334.4

0.3055

0.9617

91.93

2.879

0.9039

8.5

224.8

0.6426

0.9135

98.71

4.542

0.6152

5.6

323.2

0.06929

0.9736

39.88

2.076

0.8988

7.0

260.5

0.2778

0.9885

76.71

3.139

0.8483

8.5

202.8

0.4680

0.8334

80.92

4.191

0.5400

5.6

261.0

0.3818

0.9736

91.84

3.659

0.8095

7.0

220.7

0.0633

0.9837

24.95

2.008

0.9293

8.5

167.8

0.6409

0.9687

75.27

4.522

0.7561


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Table 2. Parameters and correlation coefficients for 8 nm bilayer coated IONPs with different coatings derived by Langmuir and Freundlich isotherm models. Langmuir Samples

pH

qmax

Freundlich

k R2

(mg/g) (L/mg)

K 1/n

n

R2

((mg/g)(L/mg) )

5.6

635.2

0.2733 0.9812

180.2

2.987 0.8584

7.0

454.3

0.8691 0.9364

185.8

3.830 0.8526

8.5

389.1

1.333

0.9619

187.5

4.649 0.8442

5.6

419.2

0.0824 0.9927

59.75

2.183 0.9465

7.0

297.1

0.3833 0.9392

112.2

3.811 0.9812

8.5

276.2

1.935

0.9443

160.4

6.021 0.8891

5.6

274.2

0.4377 0.9843

92.52

3.366 0.9189

7.0

262.1

1.681

0.8540

143.3

5.737 0.5693

8.5

236.0

0.1699 0.9913

58.41

2.887 0.8924

5.6

306.1

0.7282 0.9449

136.8

4.420 0.7466

NLS-IONPs 7.0

293.0

1.065

0.9636

143.0

4.961 0.6905

8.5

273.9

0.7189 0.9766

109.5

3.868 0.8917

5.6

217.9

1.909

0.9675

126.9

6.411 0.5632

SDS-IONPs 7.0

213.5

0.9684 0.9384

97.03

4.406 0.9303

8.5

201.9

0.0476 0.9971

18.11

1.871 0.9683

5.6

657.1

1.113

0.9282

276.6

4.148 0.7559

SDP-IONPs 7.0

416.4

1.633

0.8633

200.3

4.536 0.7911

8.5

414.3

1.881

0.8249

218.4

5.479 0.5887

OA-IONPs

SA-IONPs

LA-IONPs


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1


Table 3. Comparison of uranium sorption capacity of bilayered IONPs with other adsorbents. Adsorbent

Experimental conditions qmax (mg/g) References

Hematite

pH = 5.5, T = 25 ºC

5.59

59

Akaganeite

pH = 6.0, T = 30 ºC

90.4

60

Magnetite

pH = 7.0, T = 25 ºC

5.5

10

Iron oxyhydroxide

pH = 6.0, T = 25 ºC

278

61

Fe3O4@TiO2

pH = 6.0, T = 25 ºC

118.8

62

Fe3O4@SiO2

pH = 6.0, T = 25 ºC

52

63

Fe3O4@GO

pH = 5.5, T = 25 ºC

69.5

64

Serine-Fe3O4

pH = 3.6, T = 25 ºC

116.5

65

Amidoxime modified Fe3O4@SiO2 pH = 5.0, T = 25 ºC

105

45

PAAM-FeS/Fe3O4

pH = 5.0, T = 20 ºC

311

2

OA-IONPs

pH = 5.6, T = 25 ºC

635

this work

SDP-IONPs

pH = 5.6, T = 25 ºC

657

this work

2


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Table 4. EXAFS fitting parameters. Sample

U-Oax

U-Oeq1

U-Oeq2

U-C

U-Fe

∆E0 (eV)d

χν2e

2.0

2.88(8)

3.12

-

0.6(1)

-3(1)

0.3

-2(2)

2.0

-2(2)

2.8

-1(2)

4.4

Uncoated

Na

IONP

R (Å) b

1.759(3)f

2.238(9)

2.412(9)

-

3.40(1)

σ2 (Å2) c

0.0023(2)

0.004

0.004

-

0.008

2.0

2.8(2)

3.2

1.0(4)

0.1(3)

15 nm

N

OA-IONP

R (Å)

1.739(7)

2.28(2)

2.45(2)

2.87(3)

3.5(2)

σ2 (Å2)

0.0015(3)

0.004

0.004

0.004

0.008

2.0

2.3(2)

3.7

0.7(4)

0.4(4)

20 nm

N

OA-IONP

R (Å)

1.745(7)

2.24(2)

2.41(2)

2.86(4)

3.45(6)

σ2 (Å2)

0.0023(4)

0.004

0.004

0.004

0.008

2.0

2.6(2)

3.7

0.4(5)

0.4(4)

25 nm

N

OA-IONP

R (Å)

1.761(8)

2.24(2)

2.41(2)

2.89(8)

3.48(7)

σ2 (Å2)

0.0024(5)

0.004

0.004

0.004

0.008

4

a

5

threshold Fermi level between the data and theory.

6

parentheses represents the 1σ uncertainty in the last digit; parameters without specified

7

uncertainties were held constant during fitting.

Coordination number.

b

Interatomic distance.

c

Debye-Waller factor. e

d

Difference in the

Goodness of fit parameter

66 f

. Value in


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8


Table 5. XANES fitting results of 8 nm IONPs with varied bilayer coatings. Percent Component Samples

U(VI)

U(IV)

OA-IONPs

40

60

NLS-IONPs

69

31

SA-IONPs

69

31

SDP-IONPs

76

24

LA-IONPs

80

20

SDS-IONPs

85

15

9


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ASSOCIATED CONTENT Supporting Information. IONPs size distribution, TEM images of IONPs in water, DLS size distribution of IONPS in water with different sizes, DLS size distribution of IONPs in water with different coatings, distribution of aqueous U(VI) species, hydrodynamic size change of IONPs with different coatings in uranium solution, hydrodynamic size change of IONPs with different sizes in uranium solution, aggregation profiles of NLS-IONPs, chemical structures of surfactants, and zeta potentials of bilayered IONPs, these 8 figures and 2 tables are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed: John D. Fortner: Tel: +1-314-935-9293; Fax: +1-314-935-5464; Email: [email protected] Funding Sources The authors would like to thank the American Chemical Society’s Petroleum Research Fund (#52640-DNI10), U.S. National Science Foundation (CBET, #1236653 and #1437820), and U.S. Army Corps of Engineers (W912HZ-13-2-0009-P00001) for supporting this work. ACKNOWLEDGMENT The authors gratefully acknowledge the support from American Chemical Society’s Petroleum Research Fund (#52640-DNI10), the U.S. National Science Foundation (NSF) (CBET, #1236653 and #1437820), U.S. Army Corps of Engineers (W912HZ-13-2-0009-P00001), and the Department of Energy (DOE) Subsurface Biogeochemical Research Program (DE-SC0006857). TEM, DLS, ultracentrifugation, and ICP-MS/ICP-OES analyses were provided by the Nano


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Research Facility (NRF) at Washington University in St. Louis, a member of the National Nanotechnology Infrastructure Network (NNIN), which is supported by the U.S. NSF (#ECS0335765). XAFS measurements were performed at Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. The authors also thank Dr. Daniel E. Giammar for his thoughtful discussion during the preparation of this manuscript.


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REFERENCES 1. Sun, Y.; Ding, C.; Cheng, W.; Wang, X., Simultaneous Adsorption and Reduction of U(Vi) on Reduced Graphene Oxide-Supported Nanoscale Zerovalent Iron. J. Hazard. Mater. 2014, 280, 399-408. 2. Shao, D.; Wang, X.; Li, J.; Huang, Y.; Ren, X.; Hou, G.; Wang, X., Reductive Immobilization of Uranium by Paam-Fes/Fe3o4 Magnetic Composites. Environ. Sci.: Water Res. Technol. 2015, 1 (2), 169-176. 3. Zhang, D.; Chen, Z.; Omar, H.; Deng, L.; Khashab, N. M., Colorimetric Peroxidase Mimetic Assay for Uranyl Detection in Sea Water. ACS Appl. Mater. Interfaces 2015, 7 (8), 4589-4594. 4. Wang, Z.; Lee, S.-W.; Catalano, J. G.; Lezama-Pacheco, J. S.; Bargar, J. R.; Tebo, B. M.; Giammar, D. E., Adsorption of Uranium(Vi) to Manganese Oxides: X-Ray Absorption Spectroscopy and Surface Complexation Modeling. Environ. Sci. Technol. 2012, 47 (2), 850858. 5. Gunathilake, C.; Gorka, J.; Dai, S.; Jaroniec, M., Amidoxime-Modified Mesoporous Silica for Uranium Adsorption under Seawater Conditions. J. Mater. Chem. A 2015, 3 (21), 11650-11659. 6. Catalano, J. G.; Brown, G. E., Analysis of Uranyl-Bearing Phases by Exafs Spectroscopy: Interferences, Multiple Scattering, Accuracy of Structural Parameters, and Spectral Differences. Am. Mineral. 2004, 89 (7), 1004-1021. 7. Kushwaha, S.; Sreedhar, B.; Padmaja, P., Xps, Exafs, and Ftir as Tools to Probe the Unexpected Adsorption-Coupled Reduction of U(Vi) to U(V) and U(Iv) on Borassus FlabelliferBased Adsorbents. Langmuir 2012, 28 (46), 16038-16048. 8. Wu, P.; Hwang, K.; Lan, T.; Lu, Y., A Dnazyme-Gold Nanoparticle Probe for Uranyl Ion in Living Cells. J. Am. Chem. Soc. 2013, 135 (14), 5254-5257. 9. Kong, L.; Zhu, Y.; Wang, M.; Li, Z.; Tan, Z.; Xu, R.; Tang, H.; Chang, X.; Xiong, Y.; Chen, D., Simultaneous Reduction and Adsorption for Immobilization of Uranium from Aqueous Solution by Nano-Flake Fe-Sc. J. Hazard. Mater. 2016, 320, 435-441. 10. Das, D.; Sureshkumar, M. K.; Koley, S.; Mithal, N.; Pillai, C. G. S., Sorption of Uranium on Magnetite Nanoparticles. J. Radioanal. Nucl. Chem. 2010, 285 (3), 447-454. 11. Wang, L.; Yang, Z. M.; Gao, J. H.; Xu, K. M.; Gu, H. W.; Zhang, B.; Zhang, X. X.; Xu, B., A Biocompatible Method of Decorporation: Bisphosphonate-Modified Magnetite Nanoparticles to Remove Uranyl Ions from Blood. J. Am. Chem. Soc. 2006, 128 (41), 1335813359. 12. Li, W.; Mayo, J. T.; Benoit, D. N.; Troyer, L. D.; Lewicka, Z. A.; Lafferty, B. J.; Catalano, J. G.; Lee, S. S.; Colvin, V. L.; Fortner, J. D., Engineered Superparamagnetic Iron Oxide Nanoparticles for Ultra-Enhanced Uranium Separation and Sensing. J. Mater. Chem. A 2016, 4 (39), 15022-15029. 13. Camtakan, Z.; Erenturk, S.; Yusan, S., Magnesium Oxide Nanoparticles: Preparation, Characterization, and Uranium Sorption Properties. Environ. Prog. Sustainable Energy 2012, 31 (4), 536-543. 14. Ma, S.; Huang, L.; Ma, L.; Shim, Y.; Islam, S. M.; Wang, P.; Zhao, L.-D.; Wang, S.; Sun, G.; Yang, X.; Kanatzidis, M. G., Efficient Uranium Capture by Polysulfide/Layered Double Hydroxide Composites. J. Am. Chem. Soc. 2015, 137 (10), 3670-3677.


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Engineering Nanoscale Iron Oxides for Uranyl Sorption and

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